1. Field of the Invention
The present invention relates to power converter, and more specifically relates to a control circuit of switching power converter.
2. Description of Related Art
FIG. 1 shows a traditional power converter with a synchronous rectifier, for improving the efficiency of power conversion. A magnetic device such as a transformer 10 includes a primary winding NP and a secondary winding NS. A switch 15 is connected to the primary winding NP for switching the transformer 10 and for regulating the output of the power converter. The secondary winding NS is coupled to the output of the power converter through a power switch 20 and a capacitor 30. The power switch 20 and its body diode 25 are operated as the synchronous rectifier. A voltage VE is applied to the primary winding NP in response to the turning-on of the switch 15 during the magnetization period. Therefore, a charge current IC is generated in accordance with the voltage VE and inductance of the primary winding NP. Meanwhile, a magnetized voltage VS is produced at the secondary winding NS. Once the switch 15 is turned off, the energy of the transformer 10 is delivered to the output of the power converter through the secondary winding NS and the power switch 20. A demagnetized voltage (the output voltage VO) is thus applied to the secondary winding NS during the demagnetization period. A discharge current ID is generated according to the demagnetized voltage and the inductance of the secondary winding NS.
                              I          C                =                                            V              E                                      L              P                                ×                      T            CHARGE                                              (        1        )                                          I          D                =                                            V              O                                      L              S                                ×                      T            DISCHARGE                                              (        2        )            
where LP and LS are the inductances of the primary winding NP and the secondary winding NS of the transformer 10, respectively. TCHARGE is the magnetization period; and TDISCHARGE is the demagnetization period.
In continuous current mode (CCM) operation, the switch 15 is turned on before the transformer 10 is completely demagnetized. Under the discontinuous current mode (DCM), the energy in the transformer 10 is fully demagnetized before the start of the next switching cycle. FIGS. 2A and 2B show the waveforms of the DCM and CCM, respectively. If the power switch 20 is not turned off after the transformer 10 is fully demagnetized, a reverse current will be flowed to the power switch 20 to discharge the capacitor 30. This reverse current decreases the efficiency of the power converter. In order to avoid the reverse current, a conventional method had been proposed for the synchronous rectification, such as in “PWM controller for synchronous rectifier of flyback power converter” by Yang et al., U.S. Pat. No. 6,995,991. A resistor 40 and its control circuit 45 are used to turn off the power switch 20 once the discharge current ID is lower than a threshold value. Furthermore, a phase-lock circuit is equipped to turn off the power switch 20 before the start of the next switching cycle during the CCM operation. Nevertheless, the current detection and the phase-lock circuit produce power losses and add complexity to the system. Furthermore, a wide variable frequency system, such as a resonant power converter, causes problems for phase locking.